The present invention relates to the measurement of dynamic light scattering from particles in suspension and, in particular, to a homodyning probe for such measurements.
Dynamic light scattering (DLS) is a useful tool for measurement of the characteristics of particles in suspension. Optical scattering from a dilute monodisperse suspension is an ideal experimental configuration for characterizing a stable colloid. The performance of such measurements becomes more difficult in the following six regimes: where the sample tends to opacity, is not monodisperse, the particles are too small to scatter enough light, there is too little sample, the necessary geometry gives unwanted stray light (either coherent or incoherent flare), or the sample has non-Gaussian characteristics.
U.S. Pat. No. 5,956,139, which is incorporated herein by reference, provides significant improvements where the sample tends to opacity. Briefly stated, the patent teaches the use of cross-correlation between two spaced apart detectors to minimize the effects of multiple scattering.
U.S. Pat. No. 4,975,237, which is incorporated herein by reference, provides improvements in the other areas, i.e., the sample is not monodisperse, the particles are too small to scatter enough light, there is too little sample, the necessary geometry gives unwanted stray light (either coherent or incoherent flare), or the sample has non-Gaussian characteristics.
Substantial improvements are required in all of these regimes.
xe2x80x9cHeterodyne amplificationxe2x80x9d to improve signal-to-noise ratio (SNR) has long been used in the radio art. Analogous techniques in the optical spectrum have been exploited similarly and, because the local oscillator is typically derived from the same coherent laser source as that which illuminates the probed sample, the operation has been commonly referred to as xe2x80x9chomodynexe2x80x9d amplification. Square-law optical detectors measure intensity, the product of the incident field with its complex conjugate. The addition of a coherent optical field to the scattered field prior to detection permits the detected power to reflect the properties of the scattered electric field, rather than its customary intensity. The technique has been applied to DLS using autocorrelation, requiring only an arbitrary but stable phase for the local oscillator, and it is possible but technically challenging to build sufficiently stable apparatus, either on an optical table, or more recently using optical fibers as in U.S. Pat. No. 4,975,237. This technique does not improve the performance in the case where the sample tends to opacity. The incorporation of several techniques to address all six items simultaneously has been inhibited by the great difficulty of establishing a constant and stable phase relationship between the local oscillators mixed with the signal at each of the two detectors in U.S. Pat. No. 5,956,139, without which the measured correlation function tends to zero everywhere.
The present invention uses a novel and simple design for an optical probe which simultaneously overcomes past instabilities and makes possible the simultaneous use of homodyne detection and cross correlation incorporating all of the advantages included in the above-mentioned patents.
An optical probe for analyzing a sample illuminated by a laser includes an input optical fiber operably connectable to the laser where the input optical fiber has an entrance end and an exit end. The probe also includes a first beam splitter where the first beam splitter is adapted to transmit an alignment portion of a light beam from the input fiber exit end and to reflect a homodyning portion of the light beam from the input fiber. The probe also includes a lens between the input fiber exit end and the first beam splitter and a first and a second output optical fiber, each having an entrance end and an exit end, each exit end being operably connectable to respective optical detectors. The probe also includes a second beam splitter which is adapted to reflect at least a portion of the reflected homodyning portion into the output fiber entrance ends and to transmit light from the laser scattered by the sample into the entrance ends.